As shown, 1o responses are characterized by mainly IgM antibody and some IgG. In 2o responses, some IgM is produced (a transient "blip" that tends to decrease relatively quickly) but the main immunoglobulin produced is IgG.

Not shown is the temporary decrease in antibody level at the time of the 2o response. The decrease occurs when antibody attaches to transfused red cells. In effect, the transfused cells mop up the patient's antibody. This leads to a positive DAT and may result in the antibody screen being falsely negative until antibody production exceeds the ability of the transfused cells to adsorb them.

The key difference from the perspective of blood transfusion is as follows:

Following the first exposure to a foreign antigen, a lag phase occurs in which no antibody is produced, but activated B cells are differentiating into plasma cells. The lag phase can be as short as 2-3 days, but often is longer. Following red cell transfusion, the lag phase before detectable antibody is produced is often weeks or months.

If a second dose of the same antigen is given days or even years later an accelerated 2° or anamnestic immune response occurs. This lag phase is usually very short (e.g., 3 or 4 days) due to the presence of memory cells.

Because of the rapid nature of the 2o response, for patients who have been transfused or pregnant within the last three months, or if history of transfusion or pregnancy is uncertain or unknown, specimens for compatibility testing shall be no more than 3 days old.1

This is to protect the patient who has a weak, undetectable antibody and who may have a secondary immune response in which antibody levels rise quickly.

Note: Although policies vary, many labs have a pre-admission policy of using samples up to 14 or 21 or 28 days for crossmatching, providing (1) the patient has not been transfused or pregnant in the last 3 months, and (2) the antibody screen done on a fresh specimen (up to 3 days old) was negative.